1. Field of the Invention
This invention relates to an aircraft transparency having chemically tempered lithia-alumina-silica containing glass and to a method of making the chemically tempered glass.
2. Technology and Utility Discussion
Chemical strengthening (or “chemical tempering”) of glass involves an exchange of ions near the surface of the glass, e.g. a glass article with ions from an external source, typically a molten inorganic salt bath, to generate a zone near the surface of the glass which is in a state of compression relative to the interior portions of the glass. There are two types of ion exchange strengthening which differ substantially in theory and operation. The first type of ion exchange treatment is carried out above the strain point of the glass and has as its objective the alteration of the glass composition at the surface to lower the thermal coefficient of expansion in the surface layer. As the glass is cooled, a compressive stress develops at the surface of the glass due to the expansion differential. The first type of ion exchange strengthening is discussed in U.S. Pat. No. 2,779,136. The second type of ion exchange strengthening is characterized by treatment below the strain point of the glass. In the second type, the surface compression is generated by substituting large ions from an external source (e.g., a molten salt bath) for smaller ions in the glass. Typically, the substitution is sodium or potassium ions for lithium ions in a lithia-alumina-silica glass, e.g. of the type discussed in U.S. Pat. Nos. 3,218,220; 3,752,729; 3,900,329; 4,156,755 and 5,928,793, or potassium ions for sodium ions in a soda-alumina-silica glass, e.g. of the type discussed in U.S. Pat. Nos. 3,485,702; 3,752,729; 4,055,703, and 4,015,045.
Of the two types of ion exchange strengthening, the second (below the strain point) type is preferred for large-scale commercial use because maintaining the glass below its strain point avoids undesirable distortion defects in the glass. Of the two types of glass compositions, the lithia-alumina-silica glass compositions are preferred over the soda-alumina-silica glass compositions, and the preferred ion exchange is the sodium ion for the lithium ion. The lithia-alumina-silica glass compositions and the exchange of the sodium ion for the lithium ion are preferred because a deeper depth of ion exchange (“case depth”) in a shorter period of time at lower temperatures can be obtained.
An appreciation of the selection of the lithia-alumina-silica glass compositions and the exchange of the sodium ion for the lithium ion is had when a comparison of the ionic crystal radius of the sodium, lithium and potassium ions is made. The sodium atom has an ionic crystal radius of about 95 picometers (“pm”), the lithium atom has an ionic crystal radius of 60 pm and the potassium ion has an ionic crystal radius of about 133 pm. The lithium ion having a smaller ionic crystal radius than the sodium ion requires less energy to displace from the glass than the sodium ion, and the sodium ion having a smaller ionic crystal radius than the potassium ion requires less energy than the potassium ion to displace the lithium ion from the glass.
As mentioned above, lithia-alumina-silica glasses are available, e.g. disclosed in U.S. Pat. Nos. 3,218,220; 3,752,729; 3,900,329; 4,156,755 and 5,928,793. As can be appreciated by those skilled in the art, it would be advantageous to provide additional lithia-alumina-silica glass compositions for ion exchange strengthening, and in particular lithia-alumina-silica glass compositions that can attain a higher strength than the presently available lithia-alumina-silica glass compositions.